PEPII MAC Meeting, 14 Dec 04 Luminous Region Measurements with BaBar  Real-time Measurements  x,y,z centroids, RMS widths, x-z tilt  Offline Measurements  Bunch lengths: Comparing luminous region to other methods  Evidence for bunch lengthening  Luminous region longitudinal distribution Issues  Attempt to measure  y * with Babar SVT W. Kozanecki, C. O’Grady (Presenting), A. Roodman, J. Thompson, B. Viaud

PEPII MAC Meeting, 14 Dec 04 Introduction Luminosity  L ~  z /2 (z) 1/  L 2 = 1/  /  2 2 (x,y) Terminology: Luminous Region  Beamspot (60% syllable savings)

PEPII MAC Meeting, 14 Dec 04 Real-time Beamspot Measurement  Part of a new general framework for transmitting real-time Babar numbers/plots to PEP. Courtesy of Caltech, Babar Online Group.  8 dual-CPU Linux nodes running full Babar tracking on Bhabha and     events.  Usable in April, Subsequent changes improve statistics to 6000 events in 10 minute period. Parameter RMS Width Precision (10 minutes) x 80  m 0.9  m y 40  m 0.5  m z 7000  m 90  m dx/dz (20mrad) -0.25mrad Resolution dominated

PEPII MAC Meeting, 14 Dec 04 Understanding the longitudinal luminosity distribution  The longitudinal luminosity distribution L(z) depends on depends on  the LER & HER bunch lengths  the relative position of the collision point Z c, the LER magnetic waist Z 0,LER, and the HER magnetic waist Z 0,HER  optical parameters (mainly  * y in LER & HER + weak dep. on  +,- y ) = f(z) [JT, AR] [COG, BV]

PEPII MAC Meeting, 14 Dec 04 Sensitivity to ring optics  y * = 1 mm (1 ring)   z + = 0.25 mm Theory: Data:

PEPII MAC Meeting, 14 Dec 04 Evidence for bunch lengthening using real-time beamspot LER current HER currentLER current  z Lumi LER current  RF (LER-HER)

PEPII MAC Meeting, 14 Dec 04 Centroid/Width variation along bunch train (mm)  z L (mm) (mm) Data V L RF = 3.8 MV Data V L RF = 3.2 MV 0  Bunch number  3492 Abort gap (Before proceeding with bunch length…) LER/HER Waists Z c -motion Width variation caused by Z c motion relative to waists?

PEPII MAC Meeting, 14 Dec 04 Luminous-length dependence on Z Coll, z L,H W ~ z Coll (mm)  z L (mm) Data V L RF = 3.2 MV Why the sharp wall at low ? (mm) Observed width variation along bunch train Predicted width-dependence on Z Coll In principle the mininum of  z L can be used to determine the z location of the average magnetic waist.  z L (mm) z L,H waist

PEPII MAC Meeting, 14 Dec 04 Existing bunch length measurements Ring V RF (MV)  z (mm) Streak c.  z (mm) HOM sp. SC / HOM LER extrap’d HER All numbers at LER/HER current ratio= 1.5/1.0 mA/bunch Streak-camera measurements presented by A. FisherStreak-camera measurements presented by A. Fisher HOM measurements presented by A. NovokhatskiHOM measurements presented by A. Novokhatski

PEPII MAC Meeting, 14 Dec 04 LERHER  x /  y (nm) 30/1.0559/1.23  x (cm)  y (mm)  z (mm) 13.0/ V RF (MV) 3.8/ Parameters used in prediction z vtx (mm) 3.8 MV (LER)  z +, - = streak cam. L(z)/L max (arb. units) Prediction Data Note that the data are systematically narrower than the prediction, even before correcting for the z-centroid variation along the bunch train. 3.2 MV 3.8 MV Babar luminous region vs. streak camera

PEPII MAC Meeting, 14 Dec 04 z vtx (mm) 3.2 MV (LER)  z +, - = SC 3.8 MV (LER)  z +, - = SC arb. units Data Prediction Data Does beamspot agree with other bunch length measurements? Data are systematically narrower than the prediction at 2 RF values, even before correcting for the z-centroid variation along the bunch train.

PEPII MAC Meeting, 14 Dec 04  If one of the bunch lengths (say that of the HER) is known reliably, a measurement of L(z) distribution provides a measurement of the bunch length of the other beam (in this case that of the LER).  In addition, by measuring  z Lum with at least two different LER gap voltages and using the known gap-voltage dependence of the LER bunch length, one can in principle extract both bunch lengths (3 unknowns 2 data points + 1 constraint).  The method can be generalized to the use of several different gap voltages in LER & HER, providing experimental constraints & consistency checks.  Such an MD (proposed in June 04) requires a minimum of 2 LER and 2 HER RF-voltage settings  Each of the 4 data sets requires only 15’-30’ of Babar data taking  Other means of varying the bunch length in a “known” fashion can be exploited to the same effect – e.g. beam current.  First attempt simultaneously fitting L(z) at 2 LER RF voltages… Strategy for independent bunch length measurements with BaBar

PEPII MAC Meeting, 14 Dec 04 Can theoretical distribution can describe the shape of the data? Can theoretical distribution can describe the shape of the data?  Unable to understand L(z) with nominal beam parameters.   ~13 Z [mm] Fitting the Measured L(z) distribution Fix:  z HER = 11.6 mm  * y +/- = 10.6/10.3 mm Fit:  z LER = 11.8 mm 3.8 MV (LER)

PEPII MAC Meeting, 14 Dec 04 As before, but float magnetic waist Z-positions or  y * : As before, but float magnetic waist Z-positions or  y * :   ~2.2 Z [mm]   ~2.4 Surprising  y * /waist values. Focusing issue? Fitting the Measured L(z) distribution (II)  z LER =10.6mm

PEPII MAC Meeting, 14 Dec 04 Other Possible Explanations for Observed L(z)  Investigated:  Z-motion of centroid over macroscopic time  Z-motion of centroid vs. bunch number  Detector resolution/acceptance  In progress:  Test analysis with full Babar Monte-Carlo simulation (only fast simple simulation so far)  Validate fit using L(z) distribution from beam-beam simulations  x-y coupling near the IP: (x,y’) and (x’,y) Also looking to measure  y * more directly…

PEPII MAC Meeting, 14 Dec 04  Pioneered by Cinabro, et. al. (CLEO/CESR)  Principle:  y * information embedded in z- dependence of  luminosity  vertical luminous size  The issue: “doca” resolution - and its z-dependence  Examples of the extensive, ongoing resolution studies  * y Measurement With Dimuon Events J. Thompson A. Roodman

PEPII MAC Meeting, 14 Dec 04 Find the point of closest approach (poca) to the beam axis for each track   Distance from poca to beam axis is the doca for the track (a signed quantity)   Average doca of the 2 tracks in an event is used in the beam parameter fit   Distance (in x-y) between pocas is the miss distance: a measure of the resolution function (!). Measuring  y *: Understanding Resolution Clever idea!

PEPII MAC Meeting, 14 Dec 04 Testing the  y (z) fit Test full fit with MC generated with: 2  m <  y < 20  m MC is generated “flat” (no hourglass effect) Need to understand bias!

PEPII MAC Meeting, 14 Dec 04 Summary  Two new ideas:  A method for extracting bunch lengths using runs at different RF voltages (or beam currents)  An idea for how to determine the average location of the magnetic waists  Real-time measurements of beamspot positions, RMS values in place.  Analysis of beamspot data provides useful bunch length data for comparison with Streak Camera and HOM spectra, and evidence for bunch lengthening with increasing beam current.  L(z) distribution not understood  A promising effort to measure  y * with the Babar SVT.

PEPII MAC Meeting, 14 Dec 04 The Future  More real-time measurements:   y L from L sp and  x L  Z vtx vs. bunch number  More precise (<4ns) bunch-number time analysis to look for mini-train effects (currently ~100ns). Real-time and offline  Factor 3 more real-time statistics  Look at  x shapes for Uli to look for dynamic  Manpower limited (some expertise required).

PEPII MAC Meeting, 14 Dec 04 Spare Slides

PEPII MAC Meeting, 14 Dec 04 Bunch length measurements using HOM spectra Bunch length measurements using HOM spectra (as presented by AN on 5 Aug 04)

PEPII MAC Meeting, 14 Dec 04 Streak-camera bunch length measurements Streak-camera bunch length measurements (as presented by AF on 16 Jul 04) HER 16 MV Ring V RF (MV)  z (mm) Streak c.  z (mm) HOM sp. SC / HOM LER extrap’d HER 16.0 (16.5) (11.4) 1.06 LER 3.8 MV All numbers at LER/HER current ratio= 1.5/1.0 mA/bunch

PEPII MAC Meeting, 14 Dec 04 Z w [mm]= -3 (LER); 3(HER) -2 ; 2 -1 ; 1 0 ; 0  z L (mm) (mm) Predicted width-dependence on Z Coll, z L W, z H W

PEPII MAC Meeting, 14 Dec 04 R vs. Z (cm)

PEPII MAC Meeting, 14 Dec 04 Observed Y Resolution Pmin>0.6 GeV/c (and other cuts) All P RMS ~100um RMS ~300um Not good enough to measure 40um divergence over 10cm in Z. Sigh (Snirff en Francais).